all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/linuxdnsfight/linuxdnsfight_test.go

@@ -42,7 +42,7 @@ func TestWatchFile(t *testing.T) {
 	// Keep writing until we get a callback.
 	func() {
 		for i := range 10000 {
-			if err := os.WriteFile(filepath, []byte(fmt.Sprintf("write%d", i)), 0644); err != nil {
+			if err := os.WriteFile(filepath, fmt.Appendf(nil, "write%d", i), 0644); err != nil {
 				t.Fatal(err)
 			}
 			select {